Application Ser. No. 637,483 teaches a chromium, or chromium alloy electroplating solution, in which the source of chromium comprises an aqueous solution of a chromium (III) thiocyanate complex. Said application further describes a process of plating chromium, or a chromium containing alloy, which process comprises passing an electroplating current between an anode and a cathode in said electroplating solution.
In a preferred form the chromium (III) thiocyanate complex consists essentially of an aqueous solution of a chromium (III) aquo thiocyanate complex or mixture of complexes having the general formula: EQU ((H.sub.2 O).sub.6-n Cr.sup.III (NCS).sub.n).sup.3-n ;
where n = a postive integer 1 to 6 (Note: that the subscripts are always positive but the superscripts may be positive or negative). Complexes of this type are well known, see Inorganic Chemistry 9, 1024, (1970).
The plating from the solutions described in the above-mentioned application has many advantages over the conventional methods of plating chromium from the highly toxic chromic acid baths or from baths using toxic organic solvents. Most important of these is the removal of the serious health hazard present during plating and the fact that the effluent is easier and safer to dispose of. The present solution is less expensive, is more efficient electrically, and the useful life of the processing apparatus is much longer due to less corrosion. Significantly the deposited chromium is micro-crack free and is capable of being bent without cracking. However, it has been found that the appearance of the chromium deposits at low current densities are strong functions of the ratio of the total concentration of chromium III to total thiocyanate in the plating solution. For example, at a total chromium III to total thiocyanate ratio of 1:2, while bright chromium is deposited at current densities in the range 20mA/cm.sup.2 to 120mA/cm.sup.2, this quality falls off rapidly below 20mA/cm.sup.2 ; and in fact a black deposit is obtained at current densities less than about 15mA/cm.sup.2. This can be deleterious when plating complex shapes on which there is a wide range of current densities. It is believed that reason for the black deposit is the partial deposition of chromium from Cr(H.sub.2 O).sub.6.sup.3+ at low current densities. Previous attempts to plate chromium from this ion resulted in nonmetallic deposits.